FIG. 8 shows a typical shoe press apparatus at the press part N of a papermaking machine. In this shoe press apparatus, a pair of felts F, and an endless elastic belt B, having no air permeability, are pinched between a press roll P and shoe S. When the press roll P rotates in the direction of an arrow P′, the belt B also rotates in the direction of arrow B′. As a wet paper web W passes through the press part N between the felts F, water is squeezed from the web.
Oil is supplied to the inside of the elastic belt B to reduce friction against the shoe S.
Since the surface of the shoe S is opposed to the outer surface of the press roll P, the area of the press part N is large compared to the area of the press part in an apparatus composed of a pair of press rolls, and a greater water squeezing effect is achieved.
Therefore, this shoe press apparatus has the advantage that the energy expended in drying the wet paper web W is significantly reduced.
As shown in FIG. 9, which is an enlarged cross-sectional view showing the structure of an elastic belt B used in the above-described shoe press apparatus, the belt comprises a base member b, and high molecular weight elastic members e, which are provided on both sides of the base member b. The base member b is provided to impart strength to the elastic belt B as a whole. A woven fabric, having a warp and weft, is typically used as the base member.
The high molecular weight elastic members e, which form both the felt contacting surface and the shoe contacting surface of the elastic belt, are composed of a resin having a hardness of 70 to 98, such as urethane resin, etc.
A plurality of grooves (not shown) may be provided, optionally, on the felt contacting surface of the belt B, so that water squeezed from the wet paper web W in the press part N may be held in the grooves.
Compressed air is supplied to the inside of the endless elastic belt B to expand it into a cylindrical shape as shown in FIG. 8.
In the press part N, water, which is squeezed from the wet paper web W, moves to the elastic belt B through a felt F as the paper web W is pinched.
Although most of the water which moves to the elastic belt B is shaken off in the direction of arrow a in FIG. 8 as a result of movement of the elastic belt B, part of the water sometimes continues to adhere to the belt, and re-enters the press part. Thus, water adhering to the belt may not be squeezed adequately from the wet paper web W.
To address the problem of re-entry of the adhering water into the press part, a doctor blade has been proposed to remove the water adhering to the roll. The blade may be a metallic doctor blade, or a doctor blade having a felt impregnated with rubber or resin as disclosed in Unexamined Japanese Patent Publication No. 20697/1981.
However when these doctor blades applied to an elastic belt such as belt B, the result is not entirely satisfactory. For example, although a metallic doctor blade is highly effective in removing water from an elastic belt, it causes the elastic belt to wear out rapidly. Moreover, when the elastic belt is expanded by compressed air, it is not necessarily straight in the cross machine direction, and therefore it is difficult to ensure that the metallic doctor blade is in uniform contact with the elastic belt. There is also a risk of damage to the elastic belt caused by digging of the tip of the metallic doctor blade into to the elastic belt.
In the case of a doctor blade composed of a felt impregnated with rubber, resin, or the like, it is necessary to minimize the amount of impregnated material in order to improve adhesion to the elastic belt B. However, lessening of the amount of impregnated material impairs the shape retention of the doctor blade, allowing it to deform in use, with the result that its water removing capability deteriorates.
The invention addresses the above problems by providing a doctor blade comprising a fibrous laminate impregnated with resin, and having a void content between 50% and 80%. Preferably, the fibrous laminate includes at least one base material layer and plural fibrous layers. Where the fibrous laminate comprises inner and outer fibrous layers, preferably fibers of at least a portion of one of the outer layers capable of contacting a mating member, are thinner than the fibers of the other fibrous layers of the laminate.
With its void content adjusted to the range between 50% and 80%, the doctor blade according to the invention readily and easily adapts itself to the contacting surface of a mating member, e.g., an elastic belt, from which water is to be removed. The doctor blade also exhibits improved water removal capability, enhanced durability, and reduced abrasion of, and minimal damage to, the mating member. In addition, the doctor blade according to the invention exhibits excellent shape retention.